Currently, field emission devices (FEDs) have electron emitters which emit electrons into a vacuum region by means of an induced electric field near the surface of the electron emitter. The electric field in many instances is realized by providing an extraction electrode or gate electrode in close proximity to the electron emitter and applying a suitable potential therebetween. Emitted electrons are commonly, although not necessarily, collected by a distally disposed anode. However, in many instances, field emission devices are identified as electron emitters with only an associated extraction electrode. In the instances when field emission devices are employed as electron sources for display devices, it is desirable to effect a method to control electron emission to realize a preferred display image. For example, in order to provide an image on a viewing screen, electrons are emitted from some of a plurality of individually addressable field emission devices or some of an array of individually addressable field emission devices. However, currently, control of individual field emission devices is poor and inadequate, thereby not enabling adequate control of the field emission devices, thus effecting poor control of several parameters, such as brightness, turn on and turn off, and the like of each picture element or pixel.
It is known that by providing a select voltage between the extraction electrode and the electron emitter of the field emission device, that the electron emission from the electron emitter will be prescribed in accordance with an electric field induced at an emitting surface of the electron emitter. For a given voltage, a number of factors determine a magnitude of the induced electric field, thus the electron emission. A first factor is proximity of the extraction electrode to the electron emitter. The closer the extraction electrode is to the electron emitter for a given applied extraction voltage, the greater the magnitude of the induced electric field. A second factor that inversely relates the magnitude of the induced electric field is a radius of curvature of the electron emitting structure or electron emitter. Electron emitters formed as sharp tips, edges, or cones provide for high electric field enhancement near the emitting tip which includes a region of geometric discontinuity having a very small radius of curvature. Since these factors provide variation to each field emission device of any array of field emission devices, it is not practical to effect emission control by adjusting the extraction voltage or gate voltage between the gate electrode and the electron emitter. That is to say, the inventor has observed that the electron emissions from the electron emitters of any two field emission devices in an array of field emission devices are dissimilar because of fabrication variables. The inventor has also observed that methods currently used to compensate for these and other variations are complex and undesirable.
An alternative conventional technique employed in an attempt to effect electron emission control from field emission devices is to provide a controllable determined current source to the electron emitters of each field emission device of the array of field emission devices. By providing a controllable determined current source to each field emission device, it is not necessary to be concerned with fabrication variations because the voltage between the extraction electrode and the electron emitter will assume any required value (within the limits established by attendant voltage sources) to deliver the determined current.
However, conventional controllable determined current source techniques pose shortcomings that prevent a desired performance to be achieved. For example, each field emission device (FED) having an electron emitter has associated therewith a capacitance that must be charged each time the corresponding FED is required to emit electrons. Generally, the controlled current sources are required to provide dissimilar currents to each electron emitter of a plurality of FEDs in an array of FEDs in order to effect a gray scale capability for an image display. FEDs corresponding to pixel locations where the image display luminous intensity is desirably low will have imposed a requirement for a low electron emission and, therefore, a low determined current from the controlled determined current source associated therewith. The time required to charge the capacitance associated with the electron emitter of any FED is partially a function of the maximum available current into the capacitance. Thus, conventional controlled determined current sources that provide adequate current levels necessary for a desirable low FED emission levels do not provide adequate current necessary to charge the associated capacitance within an addressing time for that pixel.
Further, in applications employing controllable determinate current sources, the gray scale is effected by distinctly dissimilar current levels. Thus, the associated FED emitter capacitance must charge to a different level for each controlled determined current level. This is readily apparent when considering that the emission current density is a function of the voltage between the gate electrode and the electron emitter and that in order to provide a prescribed or determined current the voltage will assume a corresponding value. That is, a high current, corresponding to a high luminous level, will dictate a higher voltage than will a low current, corresponding to a low luminous level. This variation in the current available for emission and coincidentally charging of the associated capacitance provides intolerable difference in charging time of the various required electron emitter currents, and results in dissimilar electron emission characteristics at each electron emitter of the array of FEDs. Thus, providing variations that are intolerable and limit utility of this method of operation for image displays.
Accordingly, there exists a need for a method and a field emission device, and control circuitry which overcomes at least some of these shortcomings.